Prime Minister Andy Burnham has said national security “can’t come at the expense of social security” while defending the government’s approach to funding increased defence spending.

At Prime Minister’s Questions, Conservative leader Kemi Badenoch challenged Burnham over whether the government would spend 3% of GDP on defence by 2030 and how it would pay for the increase.

Burnham said the government’s commitment to defence was “absolute” and that a target date for reaching 3% would be set out in the spring. He also reiterated a commitment to reach 3.5% of GDP in defence spending by 2035.

He rejected a Conservative proposal to partly fund increased defence spending by saving at least £4bn through housing-benefit reforms. Burnham warned that such measures could create significantly higher levels of homelessness and increase the number of children in temporary accommodation.

“Yes, we do everything to support our national security, but it can’t come at the expense of social security,” he told MPs.

Burnham said the government would reduce the welfare bill through what he described as “the Labour way”: changing education, supporting young people and providing more mental health support. He highlighted the number of young people not in education, employment or training, known as “Neets”.

Badenoch criticised the government’s timetable, saying that 2035 was “a decade away” while threats were present now. She accused Burnham of failing to cut welfare spending and said the Conservatives’ approach would prioritise defence funding.

Burnham’s team later stressed that national security was “the most important duty of any government” and said he remained committed to reducing the welfare bill. However, it said that did not mean “crude cuts” that could put people into crisis and create higher costs elsewhere.

Benefits spending is forecast to reach £408.6bn in 2030/31, according to figures from the Department for Work and Pensions. The government has asked former minister Alan Milburn to examine the number of young people not in education, employment or training and is awaiting his recommendations.